The present invention relates to improvements in electrochemical cells, particularly cells having negative electrodes comprising zinc-based particles, such as in alkaline batteries.
An electrochemical cell (i.e., a galvanic cell or battery) has the following basic components: a negative electrode (sometimes called an anode), a positive electrode (sometimes called a cathode), and an ion-conductive solution (sometimes called an electrolyte) providing a path for the transfer of charged ions between the two electrodes when they are connected through an external load.
Some alkaline cells have anodes with zinc as an active element, and cathodes with manganese dioxide (MnO2) as an active element. Anodes do not have to be solid; in fact, conventional alkaline cells have a gelled zinc anode mixture. The mixture contains individual zinc-based particles suspended in a thickened liquid or gel containing a gelling agent, an alkaline electrolyte such as potassium hydroxide (KOH), and minor amounts of other additives, such as indium or bismuth (gassing inhibitors for reducing the undesirable tendency for hydrogen gas to build up inside the cell). The zinc-based particles are characterized by a specific size range, commonly indicated by the standard mesh size through which the particles pass. Typically, average anode particle sizes fall in the range of about xe2x88x9250/+200 mesh, indicating particles that pass through a 50 mesh screen and do not pass through a 200 mesh screen (the larger the screen number, the smaller the aperture size of the screen).
Common gelling agents used in anodes include carboxymethycellulose, polyacrylic acid (e.g., Carbopol 940(trademark) from B. F. Goodrich in Brecksville, Ohio, or POLYGEL-4P(trademark) from 3V in Bergamo, Italy), sodium polyacrylate (e.g., CL-15(trademark) from Allied Colloids in Yorkshire, England), and salts. Non-limiting examples of gassing inhibitors include inorganic additives such as indium, bismuth, tin and lead and organic inhibitors such as phosphate esters and anionic and non-ionic surfactants. See U.S. Pat. Nos. 5,283,139, 5,168,018, 4,939,048, 4,500,614, 3,963,520, 4,963,447, 4,455,358, and 4,195,120 for examples of various anode mixtures.
The gel anode is typically separated from the cathode by a separator, such as a thin layer of non-woven material or paper, that prohibits electronic conduction between the anode and the cathode but allows ions to pass between them.
Alkaline Zn/MnO2 cells have been commercially available for over 30 years, during which time their performance characteristics have been incrementally optimized by the industry in an attempt to provide the xe2x80x9clongest lastingxe2x80x9d battery (i.e., one with the greatest overall capacity, measured in ampere-hours) within the volume constraints imposed by the international size standards (e.g., AAAA, AAA, AA, C, D cylindrical and 9 volt prismatic sizes). The volume within such standard cells, into which the active materials are packed, is more or less fixed. The amount of energy available from any given cell size (which is a function of the total amount of the active elements in the cell) has a theoretical upper limit which is defined by the internal cell volume and the practical densities of the active components that are employed.
In addition to trying to produce the xe2x80x9clongest-lastingxe2x80x9d battery, battery manufacturers are also trying to increase the maximum instantaneous rate of electrical current that can be generated from a battery under a given load without the battery voltage dropping below a minimum value. The motivation for increasing this xe2x80x9cmaximum discharge ratexe2x80x9d capability includes the ongoing development of electronic products, such as cellular phones, which require high currents from small packages. Some of these new devices automatically test the voltage levels of their batteries, and therefore may cause the premature disposal of batteries which have remaining overall capacity, if the sensed voltage dips excessively during a period of high current draw.
When a high current is being drawn from a battery, the voltage of the battery may drop due to zinc-based particle surface xe2x80x9cpassivationxe2x80x9d or anode polarization which can indicate a localized lack of sufficient hydroxide ions to sustain the chemical reaction of the cell. It is believed that a certain amount of porosity is necessary for the free supply of OH- ions coming from the electrolyte and the free disposal of Zn(OH)4xe2x88x92, Zn(OH)2 or ZnO reaction products back into the electrolyte. If the zinc-based particles are too densely crowded, or if their surfaces are inaccessible due to accumulation of reaction products, the reaction cannot keep up with the rate of current draw. Batteries with densely packed zinc-based particles in their anodes may perform acceptably with very stable voltage levels while supplying low continuous currents, but drop to very low, unacceptable voltages when a high current is drawn due to zinc crowding (sometimes referred to as xe2x80x9cchokingxe2x80x9d or being xe2x80x9celectrolyte starvedxe2x80x9d).
In addition, too little electrolyte can starve the overall chemical reaction of the cell or cause the battery to xe2x80x9cdry outxe2x80x9d, as water from the electrolyte is continuously consumed during discharge. The net reaction inside the cell is:
Zn+2MnO2+H2Oxe2x86x92ZnO+2MnOOH.
Thus, competing with the desire to pack as much zinc-based material as possible into the available anode volume to increase overall capacity for xe2x80x9clong lifexe2x80x9d is the need to provide a sufficient amount of electrolyte to avoid xe2x80x9cchokingxe2x80x9d during periods of high discharge rate.
The invention is based upon the discovery that including very small zinc-based particles (i.e., fines or dust) among the zinc-based particles of the anode of an alkaline electrochemical cell can provide good cell performance characteristics, especially those characteristics related to high discharge rate performance.
As used herein, xe2x80x9cfinesxe2x80x9d are particles small enough to pass through a standard 200 mesh screen in a normal sieving operation (i.e., with the sieve shaken by hand). xe2x80x9cDustxe2x80x9d consists of particles small enough to pass through a standard 325 mesh screen in a normal sieving operation.
A zinc-based particle can be formed of, for example, zinc or a zinc alloy. Materials that can be alloyed with zinc to form zinc-based particles include gassing inhibitors, such as indium and/or bismuth. Generally, a zinc-based particle formed of a zinc alloy will be mostly zinc. A zinc-based particle can be spun or air blown.
The zinc-based particles can include a plating material, such as indium and/or bismuth.
As used herein, a xe2x80x9czinc-based particlexe2x80x9d refers to a singular or primary particle of zinc-based material rather than an agglomeration of more than one particle of zinc-based material. An anode can contain primary particles of zinc-based material and/or agglomerates of primary particles of zinc-based material.
According to one aspect of the invention, a negative electrode for an electrochemical cell contains zinc-based particles suspended in a fluid medium, with at least about 1 percent, by weight, of the zinc-based particles being of xe2x88x92200 mesh size or smaller. Even higher weight percentages (e.g., 6 percent, 10 percent, 25 percent, 50 percent, 80 percent, 90 percent or 100 percent) of zinc-based fines can be preferable.
In some embodiments, the zinc-based particles also include at least about 25 percent, by weight, (e.g., at least about 50 percent, 75 percent, 90 percent or 99 percent) of particles between about 20 and 200 mesh size or larger.
In certain embodiments, it is preferable that a substantial percentage (e.g., 10, 45, 80, 90 or 100 weight percent) of the zinc-based particles are dust (of xe2x88x92325 mesh size or smaller, as defined above). However, in other embodiments, less than 10 weight percent of the zinc-based particles may be of xe2x88x92325 mesh size or smaller (e.g., about 1 weight percent to about 10 weight percent, such as about 6 weight percent).
The negative electrode may include a surfactant. The fluid medium preferably includes both an electrolyte and a thickening agent.
The zinc-based particles can be spherical or nonspherical in shape. Nonspherical particles can be acicular in shape (having a length along a major axis at least two times a length along a minor axis) or of flake form (having a thickness of no more than about 20 percent of their maximum linear dimension).
According to another aspect, a negative electrode mixture for an electrochemical cell contains zinc-based particles suspended in a fluid medium with the zinc-based particles comprising less than about 68 percent (e.g., less than about 64 percent, 60 percent, 55 percent or even 45 percent) of the electrode mixture, by weight. The zinc-based particles include a sufficient proportion of particles of about xe2x88x92200 mesh size or smaller to provide an electrode resistivity of less than about 0.2 ohm-centimeters. Preferably, at least about 1 percent, by weight, of the zinc-based particles are of xe2x88x92200 mesh size (more preferably, of xe2x88x92325 mesh size) or smaller.
According to another aspect, the invention features a primary electrochemical cell having a cathode, an anode with zinc-based particles suspended in a fluid medium, at least 1 percent, by weight, of the zinc-based particles being of xe2x88x92200 mesh size or smaller, and a separator between the cathode and the anode.
The anode of the electrochemical cell may include other features, such as zinc-based particle sizes, mentioned above.
According to a further aspect, a negative electrode slurry for an electrochemical cell contains zinc-based particles suspended in a fluid medium including an electrolyte. The slurry has a resistivity of less than about 0.2 ohm-centimeters and the zinc-based particles comprise less than about 68 percent, by weight, of the slurry. The slurry can contain less than about 64 percent, 60 percent, 55 percent or even 45 percent, by weight, zinc-based particles.
According to another aspect of the invention, a method of generating an electric current includes accumulating ions on the surface of zinc-based particles suspended in a fluid medium containing an electrolyte, at least about 1 percent, by weight, of the zinc-based particles being of xe2x88x92200 mesh size or smaller.
In one aspect, the invention features a composition that includes a fluid medium and zinc-based particles contained in the fluid medium. The zinc-based particles have an average particle size of less than about 175 microns.
In another aspect, the invention features a battery that includes an anode, a cathode, and a separator disposed between the anode and the cathode. The anode includes a fluid medium and zinc-based particles contained in the fluid medium. The zinc-based particles have an average particle size of less than about 175 microns.
In a further aspect, the invention features a battery that includes an anode, a cathode, and a separator disposed between the anode and the cathode. The anode includes an active material in the form of zinc-based particles. All the zinc-based particles are of xe2x88x92200 mesh size or smaller.
Cells constructed according to the invention have displayed high tolerance for mechanical shock. They have also demonstrated high running voltages at high rate drains, low internal impedances under load, and good overall performance under various pulsed rate discharge loads.
In addition, the high proportion of zinc-based fines or dust can enable the total amount of zinc to be reduced (i.e., the cell can have a lower zinc xe2x80x9cloadingxe2x80x9d) while maintaining overall capacity on practical drains and without the typical loss in mechanical stability normally associated with a reduction in zinc loading. This is believed to be due, in part, to a high efficiency of zinc usage and good particle-to-particle connectivity.
By reducing the total zinc loading needed to achieve a given performance level, water and alkaline electrolyte can be added which may reduce the risk of anode choking.
In some embodiments, it is desirable for the anode to have a relatively low resistivity. In these embodiments, the anode can have a resistivity of about 0.2 ohm-centimeters or less.
Other advantages and features will become apparent from the following description and claims.